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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Population dynamics of the hard clam, Mercenaria mercenaria (Linne'), and its relation to the Georgia Hard Clam Fishery

Walker, Randal Leonard 05 1900 (has links)
No description available.
22

Sex ratios in terrestrial isopods

Brunet, Johanne January 1986 (has links)
No description available.
23

Seasonal changes in the physiological and hormonal aspects of reproduction in the male long-fingered bat, Miniopterus schreibersii (Mammalia : Chiroptera)

Paton, Joy Carol January 1989 (has links)
The reproductive cycle of Miniopterus schreibersii is modified as a consequence of a period of torpor/hibernation over the winter months. This bat is one of three genera which employ the reproductive strategy of delayed implantation in which the blastocyst remains free in the uterine lumen during the winter months. Spermatogenesis is initiated in February, with spermiogenesis occuring in March. Copulation, ovulation and fertilization takes place between March and May after which the testes regress and remain in an inactive condition until the following summer. Activity in the accessory gland complex is initiated in March and continues until early June. Plasma testosterone concentrations reach a peak in May, decline over the winter months and reach a second, unexplainable peak in October. Leydig cells are secretorily active in February/early March after which they undergo vacuolation and final degeneration. The question of the life cyle of Leydig cells is addressed and an increase in a certain cell after Leydig cell activity suggests that this cell type may be the precursor of Leydig cells.
24

Reproductive conflicts in honeybee colonies

Pirk, Christian Walter Werner January 2002 (has links)
In advanced eusocial hymenopteran societies workers have ovaries and can lay eggs, but are unable to mate. Workers are more related to their own offspring than to every other member of the colony. So worker reproduction contains both worker-worker and worker-queen conflict. The queen- worker conflict is discussed elsewhere, but if the queen mates with more than two males, worker policing should be selected to lower potential conflicts. However in the Cape honeybee it was predicted that worker policing is absent or less expressed than in other honeybee subspecies, because workers produce female offspring thelytokously. So laying workers and their offspring are nearly genetically identical, which results in the fact that other workers are as related to workers derived from eggs laid by the queen as laid by a worker. However, worker reproduction may be costly and therefore worker policing could be an evolutionary adaptation in the Cape honeybee to lower the costs derived from laying worker activity. Indeed, Cape honeybee colonies show efficient egg removal behaviour, suggesting that other factors like colony efficiency could favour egg removal behaviour. Since egg removal behaviour is a colony phenomenon, factors that affect colony performance could also affect egg removal behaviour. Egg removal behaviour was considerably affected by environmental changes, indicating that other tasks have a higher priority than egg removal behaviour. Thousands of queenright colonies of the neighbouring subspecies (A. m. scutellata) were taken over by laying A. m. capensis workers, showing that A. m. capensis workers are facultative social parasites. These observations strongly indicate that laying workers of A. m. capensis are able to evade worker policing and the inhibitory effects of the queen pheromones, but what potential strategies could these laying workers use to increase the survival of their eggs and evade the queen? On the one hand, egg removal behaviour is variable. One behavioural strategy of laying workers to achieve successful reproduction could be that they lay during periods with low egg removal behaviour. On the other hand, the inhibitory effect of the queen’s pheromones diminishes with distance. Maybe the level of egg removal also depends, like the inhibitory effect of the queen pheromones, on the distance from the queen. Indeed, further away from the queen the effect of the queen pheromone and the level of egg removal is reduced, making successful worker reproduction possible. In both subspecies, A. m. capensis and A. m. scutellata, egg removal behaviour is reduced further away from the queen. In the case of A. m. scutellata egg removal is lacking further away from the queen. This explains why colonies of scutellata are so prone to takeovers by laying Cape honeybee workers. One question in the context of parasitic Cape honeybees is how they manage to get into the host colony. One way could be that they get into the colonies during a natural colony merger which is common in African bees. Two unrelated colonies merged and it took them only 24 hours to show effective integration. Because both colonies are unrelated, the potential reproductive conflict among workers should be more strongly expressed than in a normal colony, which is not the result of a merger. Therefore, the effect of nestmate recognition for eggs on the egg removal behaviour was investigated. The results suggest that workers recognise the origin of an egg and that the standard policing experiments overestimate the level of egg removal and only represent relative values. Moreover, the results show that colony specific components on the eggs are more important than a postulated queen egg marking pheromone. Finally, for the first time empirical evidence from a population of the parasitic laying Cape honeybee workers, invading thousands of colonies of A. m. scutellata in northern South Africa, for a short-sighted selection theory is presented.
25

Role of Mitochondrial Dynamics and Cell Death in a Dihydrotestosterone-induced Rat Model of Polycystic Ovarian Syndrome, and its Regulation by Gonadotropin

Mazier, Hannah January 2016 (has links)
Polycystic ovarian syndrome (PCOS) is a multi-factorial infertility disorder whose etiology and pathogenesis is not completely understood. Although there is an association between dysregulation of mitochondrial fission and fusion to cell death (apoptotic and autophagic) and the pathogenesis of various diseases, this had not been reported in the context of granulosa cell death, follicular growth arrest and PCOS. Follicle-stimulating hormone (FSH), a granulosa cell survival factor, is used during gonadotropin therapy to assist in follicular maturation and ovulation in women with PCOS. Whether FSH can modulate the possible dysregulation of granulosa cell mitochondrial dynamics and cell death in PCOS had not been elucidated. Chemerin is an adipokine that has been associated with PCOS and granulosa cell apoptosis in follicle cultures. Its role in cell death (apoptotic and autophagic) of primary granulosa cells had not been confirmed. In this thesis, we investigated the dysregulation of granulosa cell mitochondrial dynamics in PCOS pathogenesis by using an androgenized rat model, and its modulation by exogenous gonadotropin. The mechanisms involved in gonadotropic regulation were investigated using primary granulosa cells. Our data suggest that increased mitochondrial fission leads to early antral granulosa cell death, follicular growth arrest and anovulation in women with PCOS. FSH can regulate the phosphorylation of mitochondrial fission protein Drp1, which may lead to its suppression of mitochondrial fission and apoptosis in PCOS. Finally, chemerin had no effect on cell death in granulosa cell cultures. These findings provide a greater understanding of the processes involved in PCOS pathogenesis and the regulatory role of FSH in granulosa cells, laying the foundation for future study into the development of potential biomarkers and new treatment strategies.
26

Immunolocalisation of, and ultrastructural changes in the LH gonadotropes of Miniopterus schreibersii and Rhinolophus capensis (Mammalia: Chiroptera) in relation to their reproductive cycles

Bojarski, Christina January 1990 (has links)
LH gonadotropes are successfully identified in the anterior pituitary of Minioptems schreibersii and Rhinolophus capensis, using immunogold labelling at the light and electron microscopy level. The gonadotropes are oval to polygonal in shape and possess numerous small secretory granules, which contain LH throughout the year. Their distribution is similar in both species, although the abundance varies slightly between species and sexes. Seasonal changes are detected in gonadotrope ultrastructure, pituitary LH levels, and plasma hormone levels, and activity pattern of LH gonadotropes generally coincide with reproductive activity in both bat species. In female M. schreibersii gonadotrope activity, pituitary LH, and plasma LH levels coincide with development, implantation and gestation. During the delayed implantation gonadotropes are inactive and and high follicular period of plasma LH levels are low, coinciding with corpus luteum inactivity and low plasma progesterone levels. Implantation coincides with increased activity of the gonadotrope activity, increased plasma LH levels, reactivation of the corpus luteum and elevated plasma progesterone levels. Activation of LH gonadotropes towards the end of hibernation may be initiated by the winter solstice, which marks the change to increasing daylength. In female R capensis gonadotrope activity and high pituitary and plasma LH levels occur towards the end of follicular development. During hibernation gonadotrope activity and plasma LH levels decrease. Ovulation coincides with increasing gonadotrope activity (which follows the winter solstice), although a preovulatory peak in plasma LH is not detected. In male M. schreibersii and R. capensis, gonadotrope activity and high plasma LH and testosterone levels coincide with spermatogenesis, except during a period of reproductive inactivity in spring in male M. schreibersii, where gonadotropes appear active and plasma LH and testosterone levels are high. The reason for this apparent activity is not known. Male sperm storage during hibernation in male R. capensis coincides with low gonadotrope activity and low plasma LH and testosterone levels. Factors initiating gonadotrope activity and hence spermatogenesis are probably decreasing daylength (summer solstice) in M. schreibersii and increasing ambient temperatures and food abundance (following the hibernation period) in R.capensis.
27

Proteins in the ovular secretions of conifers

O'Leary, Stephen James Bernard 10 April 2008 (has links)
Most conifers employ a liquid secretion originating fiom within the ovule at some point during reproduction. Although widely known, these ovular secretions have been poorly characterized. Biochemical analyses of these liquids have been limited to reports of sugars, amino acids, organic acids, and calcium. The purpose of this study was to investigate the physiological regulation of conifer ovular secretions and to further elucidate their contents. Postpollination droplet production was measured in three hybrid larch trees (Larix x marschlinsii Coaz) in relation to xylem water tension in the stem. Secretion production was not correlated to the predictable diurnal fluctuation of tree water status. The ovular secretions of this species were found to be independent of the physiological condition of the stem and are likely under the control of local structures such as the cones or ovules. The concentrations of glucose, fi-uctose, and sucrose were measured in the secretions of larch and hybrid yew (Taxus x media Rehder). In agreement with results &om other conifers, the concentrations of glucose (156 mM) and fructose (145 mM) in the larch secretion were found to be higher than sucrose (1 08 mM). The pollination droplet of yew displayed a novel pattern. The sucrose concentration in this species (23 mM) was found to be an order of magnitude higher than either glucose (2.7 mM) or hctose (2.1 mM). The ovular secretions of larch, yew, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and western red cedar (Thuja plicata Donn. ex D. Donn) were found to contain complex mixtures of proteins when examined by polyacrylamide gel electrophoresis or reversed phase high performance liquid chromatography. The proteins of larch and yew were produced consistently from tree to tree and throughout the period of secretion production. N-terminal amino acid sequencing and antibody recognition identified proteins in larch and yew samples believed to be involved in pollen germination and the promotion of pollen tube elongation. The cell wall modifLing enzyme xyloglucan endotransglycosylase (XET) was identified in the larch secretion. Immunolocalization identified cells in the apical region of the larch micropyle as the site of XET production. Arabinogalactan proteins (AGPs), known to promote pollen tube growth in angiosperms, were found in the secretions of both conifer species. AGP production in the yew ovule was localized to the nucellus. Four pathogenesis-related (PR) proteins were identified in the larch and yew ovular secretions. A lipid transfer protein (LTP) belonging to the PR-14 group was identified in the larch secretion by N-terminal amino acid sequencing. A thaumatin-like protein (TLP, PR-5) was tentatively identified in the larch sample by antibody recognition. One acidic and one basic TLP were identified in the yew secretion by tandem mass spectrometry (MSMS) sequencing of internal peptide fragments. These proteins were named TmTLPa and TxmTLPb respectively. MSMS sequencing also identified a P-1,3-glucanase of the PR-2 group in the yew secretion (TmpGlu). The cDNA coding for TxmTLPa was sequenced and assessed for heterologous protein expression. The nucleic acid sequence predicts a preprotein of 233 amino acid residues with a 28 residue export signal. The putative mature protein has a predicted molecular weight of 21.40 kDa and pI of 4.4. The deduced protein sequence contains 16 cysteine residues conserved across TLPs, and five residues that contribute to the acidic cleft of antifbngal TLPs. In order to produce Z'xmTLPa in sufficient quantities to perform bioassays, the mature sequence of this protein has been inserted into a plasmid vector for the expression of a TxmTLPa fusion protein. This report contains the first simultaneous study of ovular secretion production and tree water status, the first measurements of the sugar concentrations in the ovular liquids of L. x marschlinsii and T. x media, and the first identification of proteins in the ovular secretion of any seed plant.
28

Aspects of the reproductive biology of Breviceps

Minter, Leslie Rory 18 September 2014 (has links)
Thesis (Ph.D.)--University of the Witwatersrand, Faculty of Science, 1999.
29

Factors Affecting Testicular Growth and Development in the Beef Bull

Negrin-Pereira, Nicolás January 2019 (has links)
At present no single parameter can predict potential daily sperm production (DSP) in bulls. Sertoli cells (SC) are responsible for nourishing and providing physical support to germ cells (GC) throughout spermatogenesis. The size of the SC population established before puberty is one of the major factors determining DSP in the bull. Fine needle aspiration (FNA) is a technique used for diagnostic purposes in many species but its application to determine the size of the SC population remains unexplored. Our objectives were to compare two stains (immunohistochemistry vs. conventional HE) and three techniques to determine SC density in neonatal and prepubertal bulls [22G FNA (SMEAR), 14G needle with vacuum (CORE) and tissue section (HIS)], and the effects of three different interventions on testicular growth, development and cytology in neonates, pre and peripubertal bulls: corn supplementation during mid to late gestation of dams, the age of the dam and the administration of vitamin A. The significant correlation observed between GC and SC counts in neonatal and prepubertal bulls, highlighted the relationship between both cell populations established in the early gonad. Although no significant correlation existed between techniques for SC density, the significant relationship established between SC counts and GATA4+ cell density implies the process can be automated. Corn supplementation to multiparous dams during mid to late gestation did not have any effect on testicular gross parameters and cytology in neonatal bulls coincident with the lack of differences in fetal growth between treatments. No significant differences in scrotal circumference, testicular weight or cytology existed between prepubertal bulls from primiparous heifers or adult multiparous cows. Nevertheless, bulls from heifers had lower bodyweights at 194 days of age compared with bulls from cows. The administration of vitamin A to peripubertal bulls had no effect on scrotal circumference, testicular weight or cytology. Nevertheless, epididymis in treated bulls were significantly heavier than non-treated animals. The data generated in the present experiments highlights the need for more research to generate new interventions aiming to influence the size of the SC population in the testis and the potential daily sperm production in the beef bull. / North Dakota EPSCoR Doctoral Dissertation Assistantship (DDA) program
30

Mean fitness of long-term sexual and asexual populations of Chlamydomonas in benign environments

Renaut, Sébastien January 2004 (has links)
Populations of Chlamydomonas were maintained in a benign laboratory environment as obligatory sexual or asexual populations for five years. Sexual reproduction is expected to facilitate the elimination of mildly deleterious mutations and thereby increase the mean fitness of a sexual population relative to an asexual population (Kondrashov 1988). Fitness in competition and in pure culture was measured. In neither of the fitness assays, both in solid and liquid cultures of Chlamydomonas, was a fitness advantage of sexual reproduction seen, even though the results varied depending on the definition of fitness. I hypothesized that the effect of mutation clearance could be masked by different forces acting on the selection strain (such as an antagonistic relationship between sexual and vegetative fitness).

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